Lignocellulosic biomass is the only sustainable resource in terms of cost, availability, and scale that can be converted into liquid fuels to reduce the prevailing role of petroleum in providing energy for the world's transportation needs and to decrease the emissions of fossil-based CO2 that damage the world's climate (Studer & DeMartini et al., 2010). However, physical and chemical barriers caused by the entanglement of the main components of lignocellulosic biomass, hinder the hydrolysis of cellulose and hemicellulose to fermentable sugars (Alvira & Tomás-Pejó et al., 2010). Therefore, pretreatment steps for lignocellulosic biomass aim at breaking down the lignin structure and disrupting the crystallinity of cellulose to enhance enzyme accessibility to the cellulose during the hydrolysis step (Mosier & Wyman et al., 2005). This pretreatment of lignocellulosic biomass is a primary obstacle to a low cost biological processing (Studer et al., 2010), as it represents the second most expensive unit operation (Mosier et al., 2005). The conversion of lignocellulosic biomass to ethanol, butanol or any other liquid fuel will not be competitive until the cost of the sugars from which they are made is lowered significantly (Lynd & Laser et al., 2008). Hence, the development of more effective and economic pretreatments would be of great benefit, especially for softwood with its particular recalcitrance for hydrolysis.
Lignin, the second main component of lignocellulosic biomass after cellulose, is generally regarded as a potential energy source or as feedstock for the production of arenes due to its aromatic structure in prospective biorefinery processes. The potential of lignin as a renewable feedstock for the production of aromatic chemicals is well-known. The recovery of valuable products from lignin is from economical and ecological points of view interesting as phenolics from petro-industry represent high sale volumes and find application in a wide range of industrial processes. The production of fine chemicals (e.g. vanillin) from lignin with lower sale volumes but higher sale prices also possesses large economic and ecological potential. For those reasons, numerous attempts to depolymerise lignin into monomeric chemicals have been studied (Amen-Chen, Pakdel et al., 2001), but the breakthrough for processes that convert lignin into higher value base chemicals is still pending (Voitl, Nagel et al., 2009). While different processes for the production of pulp or chemicals from cellulose have reached industrial scales, economic processes for the production of chemicals from lignin are usually restricted to the production of relatively low cost dispersing or binding agents. There exists only one commercial process that yields reasonably high amounts of a single monomeric product (vanillin) from lignosulfonates in sulfite pulping (Voitl & Rudolf von Rohr, 2010). However, the synthetic formulation of vanillin based on petrochemical routes (via guaiacol) has almost replaced vanillin production based on lignin (Hocking, 1997). The upcoming biorefinery sector offers new opportunities for the valorization of lignin, though.
Summing up, two major drawbacks in the development of profitable biorefineries using a lignocellulosic feedstock are the lack of low capital intensive and environmentally superior pretreatment processes and an unsettled value creation of the lignin. Both issues are addressed in one step in the present invention.
Most of the approaches of lignin exploitation deal with specific lignins arising as a by- or wasteproduct from certain processes in pulping or pretreatment fractionation, that have already been chemically modified. The value creation of lignin is just started to be considered at the end of the pulping. On the other hand, pretreatments for biofuel production from lignocellulosic biomass basically aim at modifying the lignin fraction for an improved enzymatic hydrolysis but not on lignin exploitation. The invention describes an integrative approach, allowing for the valorisation of cellulose and lignin. The valorisation of both fractions is implemented from the beginning, which means already at the pretreatment of lignocellulosic biomass.
The invention describes the use of scavengers for carbonium ions that are formed in the hot water treatment of lignocellulosic biomass, which can both enhance delignification to produce a highly digestible (ligno)cellulose for enzymatic hydrolysis and allow the production of a well-defined pre-depolymerised lignin fraction to be exploited in a further reaction step.